feat(auditlog): make SiteAuditBacklogReporter cadence configurable (deferred #21)

Consolidates the reporter's poll cadence onto SqliteAuditWriterOptions instead of
the hard-coded 30 s constant, closing deferred-work register item #21 (the config-
shape cleanup its own XML doc flagged as a follow-up).

- SqliteAuditWriterOptions.BacklogPollIntervalSeconds (default 30).
- Reporter reads it via IOptions; precedence explicit-override (tests) > configured
  > 30 s default; a non-positive value falls back to the default. Corrected the
  stale "hard-code / tunable in a follow-up" class-doc.
- Cadence tests + register updated (row 21 -> Resolved).

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

docs/plans/2026-07-08-deferred-work-register.md

@@ -27,7 +27,6 @@ them and are removed from this table when that plan's task lands.
 | 17 | Unified notifications+site-calls outbox page | stillpending :118 | Explicit M9 decision to keep two pages | Operator confusion reports |
 | 18 | Folder drag-drop | same, [PERM] | Permanently closed; menu reorder shipped | — (closed) |
 | 19 | Bundle signing / cluster-to-cluster pull / differential bundles | transport-design :402 | v1 manifest hash + AES-GCM held sufficient | Non-repudiation requirement across orgs |
-| 21 | SiteAuditBacklogReporter threshold consolidation | SiteAuditBacklogReporter.cs:28 | Config-shape cleanup only | Next SqliteAuditWriterOptions change |
 | 22 | KPI history hourly rollups | Component-KpiHistory.md | YAGNI; 90-day retention bounds table | KpiSample query latency on dashboards |
 
 ## Resolved (verified against the code 2026-07-10)
@@ -41,6 +40,7 @@ Rows removed from the Deferred table above once confirmed shipped. Kept here for
 | 15 | BrowseNext final-page signal not surfaced | Already surfaced (M7 browse work): `RealOpcUaClient` sets `Truncated=false`/`ContinuationToken=null` on the last page; `BrowseNodeResult` carries both; `TreeRow.razor` renders "Load more" only when a continuation token remains — no wasted BrowseNext. |
 | 16 | StubOpcUaClient throws on browse | Already resolved: `StubOpcUaClient` supports browse + address-space search, covered by `StubOpcUaClientBrowseTests`/`StubOpcUaClientSearchTests`. |
 | 20 | Deployment EXPIRED-row purge | Already resolved (PLAN-04): `PendingDeploymentPurgeActor` central singleton (spawned in `AkkaHostedService`) ticks `IDeploymentManagerRepository.PurgeExpiredPendingDeploymentsAsync` every `CommunicationOptions.PendingDeploymentPurgeInterval` (default 1h), options-validated, tested. |
+| 21 | SiteAuditBacklogReporter threshold consolidation | Shipped 2026-07-10: `SqliteAuditWriterOptions.BacklogPollIntervalSeconds` (default 30) now drives the reporter's poll cadence; explicit ctor override still wins (tests), non-positive falls back to the 30 s default. Cadence tests added; stale "hard-code / follow-up" class-doc corrected. |
 
 ## New deferrals from review 08 (this plan)
 | Item | Rationale | Revisit trigger |